We have found that nerve and muscle cells from embryonic frogs can be stimulated to differentiate and can be guided in their growth direction when grown in culture in the presence of minute electrical fields. Using the vibrating probe, we have also found currents, discover their origin and determine their ionic components, starting from the assumption that they are driven by a sodium pump in the epithelium that surrounds the embryo. Further, we plan to map the electrical fields within the embryo that result from the measured current. This will be done using pairs of connectional KCl-filled electrodes. The object will be to determine if the developing nerves and muscle in the embryo are subjected to electrical fields that are comparable to those fields that we find effective in vitro in guiding growth. We also plan to continue our study of electrical currents associated with cleavage in amphibian eggs, again using the vibrating probe.